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260 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, JUNE 1967

A COMPACT70MW, 250KV MODULATOR USING THYRATRONS

C. Latham H. Menown N.S. Nicholls Vickers Ltd. English Electric Royal Radar Radiation and Nuclear Valve Co. Ltd. Establishment Engineering Division

SUMMARY Pulse Repetition Rate Up to 1000 p.p.s. max. (at 0.5nsec flat-top) Pulse A compact oil immersed modulator is described Flatness *0.5% over specified suitable for driving a 30MWoutput klystron and flat-top. Pulse-Pulse using two 70KV 2,500 Amp hydrogen thyratrons as Time Jitter *5ns max. switches. The design is a development of an H.T. Stabilisation Better than f0.28 for earlier modulator using a 70KV triggered spark *3% mains variation. gapa This earlier model was limited to a maximum Response time less than repetition rate of about 250 p.p.s. but the 2 supply cycles. thyratron switches permit operation to over 1000 The modulator p.p.s. and a maximum duty cycle of .OOl. (Fig.11 is contained in a steel tank approximately Complete oil immersion in a single tank results 15ft. long by 3ft.3in. wide by 4ft.8in. high and in compactness, economy, reliability and facil- at one end supports and accomm- odates the klystron, the cathode itates the use of a high charging voltage which, bushing of which in turn, is lowered into the tank. . . leads to a simplification in the pulse circuit, particularly in relation to pulse flatt- Two free ening and pulse length selection. A high current standing racks housing the klystron focus supply silicon clipper is described which is used to and the klystron cooling heat exchanger are reduce the inverse voltage and which still installed in close proximity to the modulator. Other external permits rapid thyratron recovery time. items are the modulator control rack, which contains the instrumentation, INTRODUCTION trips and interlocks, and a separate motorised 1OOkVA a.c. regulator which can be installed in any convenient position. The design was commenced early in 1964, initiallyti fulfil the requirements of linear POWERSUPPLY accelerators for the Universities of Toronto and Glasgow. Although the electrical performance In the case of the TV2011 Klystron the called for new development, the physical arrange- values of pulse forming network impedance and ment was based on practice established for a pulse transformer ratio (15 ohms; 1:8) have previous generation of modulators having all been chosen such that for normal perveance tubes operating components immersed in a single oil filled tank. at 250-260kV the H.T. supply is 31-32kV. The power supply is designed Experience had shown the advantages to operate from 18kV (minimum) of this to cope with constructional form to be:- switch-on conditions, and may be run up to a maximum of 37.5kV to allow for end-of- life klystrons which 1. Excellent reliability resulting may require pulse voltages from up to 260kV. elimination of multiple high voltage bushings. It 2. Compactness. is necessary that the power supply is ad- justable over 3. Economy in manufacture. this range and that at any setting it shall be stabilised automatically. A dual 4. Simplicity of cooling and screening. system is used, one part provides some 95% of the total power and The use of a high charging is controlled by a continuously voltage, in the variable voltage regulating region of 70KV, for the transformer, while pulse forming network was the other part also retained consists of a trimming circuit from existing spark switch supplying modulator the remainder. The trimmer is fed from practice. a relatively fast acting a.c. controller employingof thyristors, the conduction angles of Modulator Specification which are determined by a circuit which compares a sample the H.T. voltage with a reference The main parameters voltage propor- are as follows:- tional to the chosen working level. Each of the Peak Power Output 70MW max. (57.514W two power supplies employs a three phase trans- normal running level former and full wave rectifier, the combining for TV2011 klystron) being done by series connection of the d.c. Mean Power Output 70kW. outputs. Flat-top Pulse Lengths 4.5 or 3.5,2.1,0.5~s selected by 3 position switch.

PAC 1967 LATHAM: COMPACT 70 MW, 250 KV MODULATOR USING THYRATRONS 261

This system provides fast compensation over a thyristor control for the contactor opening coil. range covering the working level and does not The thyristor logic includes duplication for suffer from three disadvantages, associated with reliability and easy manual check out by pusn full power thyristor control:- button.

a. If controlled rectifiers are used, either The Trimming System as primary a.c. controllers, or is actual rectifiers on the secondary of the trans- The trimming system and H.T. supply is shewn former, the d.c. output contains a high in Fig.2. The H.T. line is monitored by a poten- amplitude commutation spike except when tiometer immediately after the rectifiers, to the firing angles are set near to maximum avoid the phase shift and time lag of the L-C output. filter. It was found necessary to shield this potentiometer from stray fields, it consists of a In the dual arrangement the trimmer chain of resistors arranged approximately non- normally operates at half voltage output inductively in a cylindrical can 23" by 5" dia. and increases or decreases to compensate contained within the modulator tank, the output for mains variations but at no time is being brought out by a double-screened cable. the spike output sufficient to present a The d.c. sample is compared with a reference serious smoothing problem, tne L-C potential derived from a potentiometer mecnan- smoothing circuit (15ii, 2uF) being common ically ganged to the main a.c. regulator and fed to the sum d.c. of both rectifiers. from a stable d.c. source. Thus, as the regulator is raised the reference rises and the sample from b. Chopping the entire modulator input at the li.T. line should rise proportionally, Any levels in the range 50-100 kVA may lead difference constitutes an error signal which is to distortion of the supply waveform and fed via a chopper type d.c. amplifier to a three- when several modulators are run at phase thyristor controller to advance or retard slightly different conduction angles from the thyristor firing angles. The a.c. regulator a common source there is a risk of mutual runs up in about 25 set until the drive motor is interference with critical ancillary stopped by a relay circuit which also controls equipment. visual indicators on the modulator control panel and main accelerator control station. These The dual circuit has the advantage that relays are controlled by a bistable transistor because the greater part of the power in- circuit which sense both the reference potential put consists of a full cycle load the and the modulator mean current. Thus the H.T. supply waveform is relatively clean. runs up to a predetermined level irrespective of the load current imposed by widely different duty c. A tllyristor 'firing through' (due perhaps cycles. to breakdown or pick-up of spurious trigger) may lead to sudden increase in As the regulator runs up, the load current modulator output endangering the r.f. increases approximately linearly and the 11.T. tube. In the event of such a fault in volta,;e sags slightly due to power supply impe&rze the thyristor trimming system the total (mainly transformer reactance). A sample of the output voltage cannot rise more than a modulator mean current is fed to the error signal few percent and serious malfunction of amplifier to ensure that the trimming circuit the modulator is avoided. operates in the centre of its range at all levels and at all duty cycles. During modulator tests, The main d.c. supply has been designed with a rig consisting of three large wattage resistors, sufficient margin to permit full power operation shunted by a three pole contactor, is inserted in of the klystron without the trimmer if necessary. the mains supply line to produce, at will, a step of 3% in the input voltage and with the trimmer kkctification and Smoothing. working a change of less than 0.2% is seen on the klystron voltage pulse For both the main and trimring rectifiers strings of 1ijOOV silicon avalanche diodes are PULSE CIRCUIT employed. Over-current protection in the event of short circuit is provided by fast fault sens- Charging, ing to open the main contactor (common to both circuits). Also 9% reactance has been introduced The resonant charging system is connected to into the supply transformers to ensure that on the 'back end' of the p.f.n. in order to reduce short circuit the d.c. drawn from the rectifiers the stray capacitance seen by the switch tubes. does not exceed 28 Amps. The I't rating for the (Fig.3.) A single choke of 3.OH is used for all diodes permits this current for a period of repetition rates, the necessary hold-off being lOOm.sec following normal full load current, but achieved by a stack of silicon diodes consisting the circuit is arranged to clear in about 50m.sec, of 160 devices each rated at 1OOOV. Each diode is thereby giving a 2:l safety factor. shunted by a capacitor and resistor but selected low leakage diodes were chosen to allow the shar- To achieve this it was necessary to make ing resistors to be as high as possible, thereby tests of contactor opening times and to devise reducing the back leakage and charge loss at low

PAC 1967 262 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, JUNE 1967

repetition rates. by careful arrangement of the windings around several d.c. biased uncut loops of 0.002" hyper- Pulse Forming sil, an arrangement designed for consistent performance over long periods, in that repeated With modulators designed to produce very flat thermal cycling will not affect the core cha- pulses it is not unusual to make the mean pulse racteristics as can be the case with gapped cores. length longer than the flat-top required, by a The secondary winding is tapped to provide 35 and factor of perhaps 2. This is an advantage in 40kV outputs for the linac gun if required. that heavy despiking can be used to round off the leading edge without exciting the resonances CONTROLOF THYRATRONINVERSE VOLTAGE attributable to stray reactances. Alternatively, if the despiking is less severe, the first part During initial thyratron tests an inverse of the pulse top - having some ripple may be diode circuit was connected across the thyratron ignored. That these methods are wasteful of to stabilise the resonant charging circuit power is of little significance in low P.R.F. against variations of load impedance. Its effec- applications. In our case the maximum possible tive forward resistance was about 400 ohms. When P.R.F. was required at each pulse length but thyratrons were first operated at low repetition because of klystron mean power limitations it was frequencies, arc-back was frequently encountered not permissible to waste any appreciable part of when the peak anode voltage was raised to the the pulse. region of 60kV. There was also evidence of un- expectedly high thyratron dissipation when the At 3.511~ flat-top the mean pulse length is P.R.E. was raised. Excessive inverse current was only 4~s and the P.R.F. 250 p.p.s. Nevertheless suspected. The peak inverse voltage was found to a departure from flatness of about 20.5% is be about one half of the peak forward anode achievable over the specified flat-top by careful voltage. (Fig.4.) Also the distribution of adjustment of the P.F.N., the use of minimal de- inverse voltage between the two thyratron gaps spiking and a pulse transformer designed for low was observed during the period immediately leakage inductance. The latter is important following the pulse and it was found that inverse since one of the main causes of ripple during the voltage appeared initially only on the gap adja- start of the pulse is thought to be due to res- cent to the anode. onance of the leakage inductance and the klystron cathode/ground capacitance; reduction of the An analysis was therefore made, both leakage substantially reduces the amplitude and theoretically and by means of an electrical scale duration of the oscillation. model of the origin of this inverse voltage. Both methods indicated that the principal cause The P.F.N. is rated for continuous operation was the leakage inductance of the pulse trans- at 72kV and consists of two parts: one has five former but that reducing it within the range of sections for the short pulse (0.5psec) and the practicability, would have the effect mainly of other has eleven sections for the long pulse reducing the duration of the inverse voltage (3.5usec) tapped at the seventh for the medium rather than its peak value. It was therefore pulse (2.lusec). The inductive element consist decided to fit a low impedance end-of-line of self supporting coils arranged horizontally clipper circuit and to modify the pulse trans- above the capacitor banks. The coils have tap- former so as to reduce the mean dissipation in ping clips which permit individual adjustments the inverse diode load resistor from 8kW to 3kW. for inductance and mutual coupling. To overcome difficulty with thyratron recovery, a non-linear C.R. network was used with an Change of pulse length is accomplished by a extended foil paper capacitor. specially designed three position switch which selects either the short line or the long line With a diode load resistor of 4 times Z, and in the latter case puts seven or eleven (60 ohms), which was as low as was considered sections in circuit. Contacts are included which safe with the avalanche silicon diode stack, the modify the seventh section inductance according thyratron inverse voltage was reduced to the to whether it is the end section for the medium region of 20kV. At this level, operation free pulse or the seventh section of the long pulse, from arc-back was obtained. to give optimum flatness for each. The contacts also route the charging feed and end-of-line DEVELOPMENTOF THE SWITCHING THYRATROW inverse diode clipper to the back end of the selected portion. The body of the switch is Development has been concentrated on the constructed of 3" thick acrylic sheet and is per- CX116B thyratron. When the first tube was op- haps best described by its resemblance to a large erated there was difficulty in withstanding the slide rule about a metre in length. The sliding full H.T. voltage on the short pulse length at member is operated by a lever from above the lid high gas pressures even at low P.R.F. Honitoring of the tank. of the potential of the gradient-grid, which was connected to the centre of a high resistance Pulse Transformer potentional divider between anode and cathode, revealed that its potential rose during the early A leakage inductance of about 2uH is achieved part of each charging cycle to almost the peak

PAC 1967 LATHAM: COMPACT ‘70 MW, 250 KV MODULATOR USING THYRATRONS 263 anode voltage and later fell back to its proper Normal Fault half-way level. This was due to insufficiently Peak Current 4CQA for Ius 1OOOpps. 75k rapid recovery of the top gap, and was confirmed 250A for 5~s 250~~s. l'+OOA 5y negatively biassing the upper part of the gradient grid with respect to the lower. In Peak Inverse Voltage 75kV 120kV approx tests carried out before the introduction of the end-of-line clipper, excessive grid heating was Protective circuits limit the fault condition observed even when the valve envelope was effec- to isolated pulses separated by several seconds. tively cooled by air-blast. As a result, in a typical case, the reservoir range fell steadily The diode stack is immersed in oil in the with increase of H.T. voltage and P.R.F. until at main modulator tank, where the maximum temperature 6OkV, 500 p.p.s. satisfactory operation was is 50°C. On the basis of pulse tests on individ- obtained only at a unique reservoir setting. The ual diodes, Mullard BYX-25-EOOR was found to be inverse voltage immediately following the pulse the most suitable controlled avalanches diode. A was reduced by the introduction of the end-of- stack of 100 such diodes having 1.2kV avalanche line clipper and the design of the thyratron was voltage was judged adequate. It was desired to modified to reduce the recovery time of the avoid the need for sharing capacitors in the gradient grid and to improve its cooling. interests of economy and reliability, However, Operating at up to 1000 p.p.s. was then possible. calculations indicated that capacitance to earth However, at high P.R.F. severe corona developed would lead to excessive average avalanche on the outside of the top ceramic. It was there- dissipation in some diodes unless the stack was for decided to immerse the valve in transformer screened. oil. This also solved the cooling problem by natural convection, with a consequent gain in Using a cylindrical geometry resistance reliability. network analogue to solve Poisson's equation, it was found possible to design end-caps which would The CX1166 thyratron is used at 70kV, 2300A, ensure freedom from avalanching during the normal 50 p.p.s., lus mean. One tube is used at pulse charging cycle (Fig.6.). lengths up to 5 microseconds at 2300A peak current, with an average current limit of 2.5A. Diode stack of this design have given Between 500 and 1000 p.p.s. two thyratrons are satisfactory service in the modulators, both on used switched alternately. resistance load with intermittent short-circuits applied for a few successive pulses and with The design of the tube alleviates dissipation klystrons as loads. problems at the grids because the control grid G2, which is heated by the cathode, receives no inverse dissipation. On the other hand, the gradient grid, which is subject to inverse dissipation, is about 3" away from the control grid. This permits the introduction of a copper structure to conduct heat away from the centre of ACKNOWLEDGEMENTS the gradient grid effectively. Fig.5. The authors wish to thank the Directors of DEVELOPMENTOF AN IHVEKSE DIODE STACK Vickers Ltd., and the Managing Director of the USING CONTROLLEDAVALANCHE SILICON English Electric Valve Co., for permission to DIODES publish much of the information contained herein which is based on a similar paper originally The performance required of the diode stack presented at the Ninth Hodulator Symposium, is as follows:- Fort Monmouth, May, 1966.

PAC 1967 264 IEEE TRANSACTIONS ONNUCLEARSCIENCE,JUNE1967

MMxImm rooid400v+ ---,. REUrn.IosMOTORDRI “$Nk”4 yjjl -4; L ‘Km*---.-.36F$’ 4--- -F”’j ,+~~~~~~I~~315 k” m4,

\;; tl ~ g;is,eb&L& i ‘\\ - ~~~- CT13 ‘\ RFFEREN:F -J I SAMPLEA-

Fig. 1. View of modulator (at Glasgow). Fig. 2. Modulator Power Supply.

CHARGER 18-36KV 2 u DC- 35 -7OKV PEAK PFN 15n CLIPPER

2.2Kl-t 50n

I “0.2pF kV 140 -280KV t-y>

TIME

Fig. 3. iCIodulator Circuit. Fig. 4. Thyratron Anode Volts.

PAC 1967 LATHAM: COMPACT 70 MW, 250 KV MODULATOR USING THYRATRONS 265

I--- 6

DELAYED KEE

DELAY

1INPUT PULSE

Fig. 5. CX1168 Thyral :ron.

Fig. 6. Diode Assembly.

PAC 1967